Remove hard-coded absolute URLs from all frontend assets

What does this MR do?

addresses omnibus-gitlab#1874 (closed) by removing any reliance on hard-coded URLs from our frontend assets

Are there points in the code the reviewer needs to double check?

make sure there's nothing that I've missed which would break in production when GitLab is installed at a relative url (i.e. http://example.com/gitlab/)

Why was this MR needed?

Node is currently a production environment dependency because of this one feature, and this change should allow us to remove it.

See omnibus-gitlab#1874 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

omnibus-gitlab#1874 (closed)